SFTB NSC [National Semiconductor], SFTB Datasheet - Page 6

• Manufacturer Part Number: SFTB
• Description: Thin SOT23 1A Load Step-Down DC-DC Regulator
• Manufacturer: NSC [National Semiconductor]

www.national.com
Application Information
current. Therefore, it is recommended that V
greater than 2.5V above V
V
5.5V
When the LM2734Z starts up, internal circuitry from the
BOOST pin supplies a maximum of 20mA to C
current charges C
switch on. The BOOST pin will continue to source current to
C
0.76V.
There are various methods to derive V
1. From the input voltage (V
2. From the output voltage (V
3. From an external distributed voltage rail (V
4. From a shunt or series zener diode
In the Simplifed Block Diagram of Figure 1, capacitor
C
NMOS switch. Capacitor C
V
NMOS control switch is off (T
equals V
which the current in the inductor (L) forward biases the
Schottky diode D1 (V
across C
When the NMOS switch turns on (T
to
forcing V
at V
which is approximately
for many applications. Thus the gate-drive voltage of the
NMOS switch is approximately
An alternate method for charging C
the output as shown in Figure 3. The output voltage should
be between 2.5V and 5.5V, so that proper gate voltage will
be applied to the internal switch. In this circuit, C
provides a gate drive voltage that is slightly less than V
In applications where both V
5.5V, or less than 3V, C
from these voltages. If V
C
voltage by placing a zener diode D3 in series with D2, as
SW
BOOST
BOOST
IN
BOOST
. During a normal switching cycle, when the internal
BOOST
should not exceed the maximum operating limit of 5.5V.
>
V
V
until the voltage at the feedback pin is greater than
BOOST
and diode D2 supply the gate-drive current for the
can be charged from V
IN
BOOST
BOOST
BOOST
is then
minus the forward voltage of D2 (V
FIGURE 3. V
V
BOOST
= 2V
is
to rise thus reverse biasing D2. The voltage
– V
V
SW
BOOST
SW
IN
- V
= V
– (R
FD1
>
SW
2V
V
IN
BOOST
IN
OUT
to a voltage sufficient to turn the
2.5V for best performance.
IN
BOOST
). Therefore the voltage stored
SW
IN
= V
DSON
and V
– (R
OFF
- 0.2V
- 0.4V
IN
IN
Charges C
for best efficiency. V
IN
OUT
)
) (refer to Figure 2), V
and V
DSON
cannot be charged directly
IN
x I
OUT
is charged via diode D2 by
- V
BOOST
)
L
or V
ON
FD2
) – V
are greater than 5.5V,
x I
), the switch pin rises
OUT
BOOST
OUT
+ V
L
BOOST
is to connect D2 to
),
FD2
(Continued)
are greater than
FD1
minus a zener
:
+ V
EXT
BOOST
FD2
FD1
BOOST
)
BOOST
), during
BOOST
BOOST
20130308
. This
OUT
be
–
.
6
shown in Figure 4. When using a series zener diode from the
input, ensure that the regulation of the input supply doesn’t
create a voltage that falls outside the recommended V
voltage.
An alternative method is to place the zener diode D3 in a
shunt configuration as shown in Figure 5. A small 350mW to
500mW 5.1V zener in a SOT-23 or SOD package can be
used for this purpose. A small ceramic capacitor such as a
6.3V, 0.1µF capacitor (C4) should be placed in parallel with
the zener diode. When the internal NMOS switch turns on, a
pulse of current is drawn to charge the internal NMOS gate
capacitance. The 0.1 µF parallel shunt capacitor ensures
that the V
Resistor R3 should be chosen to provide enough RMS cur-
rent to the zener diode (D3) and to the BOOST pin. A
recommended choice for the zener current (I
The current I
current of the NMOS control switch and varies typically
according to the following formula:
where D is the duty cycle, V
I
anode of the boost diode (D2), and V
forward voltage across D2. Note that this formula for I
gives typical current. For the worst case I
current by 25%. In that case, the worst case boost current
will be
R3 will then be given by
For example, let V
= 1mA, and duty cycle D = 50%. Then
BOOST
FIGURE 4. Zener Reduces Boost Voltage from V
R3 = (10V - 5V) / (1.25 x 4.3mA + 1mA) = 787Ω
R3 = (V
is in milliamps. V
I
BOOST
I
BOOST
BOOST
BOOST
IN
= (0.5 + 0.5) x (5 - 0.7) mA = 4.3mA
I
BOOST-MAX
voltage is maintained during this time.
- V
= (D + 0.5) x (V
IN
(V
(V
ZENER
INMAX
into the BOOST pin supplies the gate
= 10V, V
INMIN
ZENER
) / (1.25 x I
– V
– V
= 1.25 x I
ZENER
ZENER
D3
D3
is the voltage applied to the
)
)
ZENER
>
<
and V
= 5V, V
1.6V
5.5V
BOOST
BOOST
– V
BOOST
D2
D2
D2
D2
are in volts, and
+ I
ZENER
is the average
) mA
= 0.7V, I
, increase the
ZENER
20130309
) is 1 mA.
)
BOOST
BOOST
ZENER
IN